1. Field of the Invention
The present invention relates to a wafer processing method for performing grinding processing while leaving a thick reinforcing rib area at the outer circumferential edge of a wafer.
2. Description of the Related Art
Semiconductor chips used in various electronic devices are generally manufactured by the following method. The front surface of a disk-like semiconductor wafer is sectioned along predetermined dividing lines into lattice-like rectangular areas. Electronic circuits are formed on the front surfaces of the areas. Then, the semiconductor wafer is thinly ground from the rear surface and divided along the predetermined dividing lines. Incidentally, electronic devices have significantly been reduced in size and in thickness in the recent years. Along with this, the semiconductor chips are required to have a smaller thickness. Therefore, the semiconductor wafer is needed to be thinner than ever before. However, if the semiconductor wafer is simply thinned, rigidity is reduced, which poses a problem in that the wafer becomes difficult to handle and becomes fragile in the step after the thinning.
To avoid such a problem resulting from the thinning, only a circular device area formed with semiconductor chips is thinly ground from the rear surface side and an outer redundant area on the periphery thereof is left as a relatively thick reinforcing rib area (see e.g. Japanese Patent Laid-open Nos. 2004-281551 and 2005-123425). In this case, since the rear face side is ground, the thick reinforcing rib area projects toward the rear surface side so that the wafer becomes concave in cross-section as a whole. Such a wafer is arbitrarily called “the drum wafer.”
In order to prevent trouble such as breakage or the like from occurring in the device area of the drum wafer or in individualized chips, it is necessary to finish the rear surface corresponding to the device area with a grinding stone having a grinding grain diameter as small as possible so as not to leave processing damage to the wafer. However, the wafer with the original thickness may be subjected to grinding processing with a grinding stone composed of fine grinding grains. In such a case, there is a problem in that not only time required for processing lengthens to lower productivity but also the grinding stone wears out fast to increase consumable tool expenses. To eliminate the problem, the drum wafer is roughly ground to some extent before being finish-ground with a grinding stone composed of fine grinding grains (see Japanese Patent Laid-open No. 2007-173487).
However, the rough grinding performed before the finish-grinding is processing whose processing rate is made faster to reduce processing time. Therefore, during the rough grinding, a burst chipping with a size of several hundred μm occurs at several positions in the inner circumferential edge of the outer circumferential reinforcing rib area. In the wafer processing of this type, the drum wafer is reduced in thickness and then stress release is performed by spin etching or the like to remove a fractured layer resulting from the grinding. During such etching, the etching may proceed from the chipping occurring position toward the radially outer circumferential side in accordance with centrifugal force to form a concave portion and cause irregularity in the outer circumferential reinforcing rib area. Thus, while the drum wafer is transferred to a subsequent step with the outer circumferential reinforcing rib area sucked and held, leak occurs at the concave portion to disturb normal suction and holding, thereby causing a transfer error.